The invention concerns a safety brake mechanism of a driven winding drum of a hoisting machine. The invention is particularly useful for braking the winding drum to a standstill without jerking, when the lowering speed is excessive, by a brake ring. The brake ring revolves concentrically with the winding drum and is frictionally joined to the latter, as soon as the brake ring lying against a cylindrical brake surface of the hoisting machine is locked. The invention also concerns a method for grinding the safety brake.
Safety brake mechanisms are familiar and are used, for example, as a safety mechanism in the form of a second brake for cable winches, sliding door drive systems, or similar winding mechanisms. Such winding mechanisms require, besides the usual service brake, a safety brake mechanism working independently of the motor brake generally arranged on the motor. Such a safety brake mechanism, upon failure of the lifting drive, for example, after gear breakage in the lifting gearbox, is intended to safely prevent the load from dropping, or terminate its unintentional descent after a relatively short drop.
One safety brake of the above-described kind is known, for example, from the German Patent DE 31 37 523 C2. It is used to safeguard a hoisting machine and is supposed to brake the cable drum without jerking when the lowering speed is excessive. For this purpose, a cylindrical brake surface is formed at the periphery of the cable drum in the region of one of its two end faces and this is surrounded by a brake ring. The brake ring is formed from individual segments and provided with a brake lining on its inner circumference, which can be pressed against the brake surface while being adjustable in its contact pressure. In the operating state of the hoisting mechanism, the brake ring is frictionally entrained by the brake lining and revolves along with the cable drum of the lifting mechanism. On its outer circumference, the brake ring is configured as a ratchet, whose teeth can engage with an outside-activated pawl, attached to the frame of the safety mechanism, which instantly locks the brake ring when there is trouble with the lifting mechanism. Since the brake ring lies frictionally against the brake surface by its brake lining, when the brake ring is locked abruptly by means of the pawl, the brake lining of the locked brake ring at first slips somewhat on the brake surface until the lifting drum is brought to a standstill. In any case, a gentle braking of the cable drum requires a properly adjusted contact pressure of the brake lining against the brake surface.
In order to properly adjust the contact pressure to the required braking moment for the braking of the cable drum, it is customary to grind the brake lining under excess load. For this, the lifting mechanism is at first completely assembled, so that the operating conditions of the subsequent working under load can be created. The actual grinding process is very time consuming, because several braking processes have to be carried out under load and, after each braking process, the tube of the winding drum has to cool down to ambient temperature. As a result of this multiple repetitive process, the production flow of the lifting mechanisms is greatly disrupted, and the time it takes the lifting mechanism to move through the final assembly becomes substantially longer, as compared to the assembly of a lifting mechanism without such an additional safety brake.
An improved solution as compared to the aforesaid prior art is described in the German Patent DE 196 33 836 C1. In this solution, the drum for the cable can be connected to the output shaft of the hoisting transmission. The output shaft is provided with a neck piece, which serves as a braking surface for the brake shoes of the safety brake. In this solution as well, blocking means are provided for halting the brake shoes, which are mounted on a brake ring, being secured either directly or indirectly to the frame, as described at the outset.
Since the brake surface for the brake shoes of the safety brake is joined directly as a structural unit to the output shaft of the lifting gearbox, the safety brake after being assembled can be adjusted to the lifting gearbox and tested without having to outfit the lifting mechanism completely with the cable drum.
Although this solution also represents an improvement over the prior art described at the outset, nevertheless it comes with the drawback that the lifting gearbox is required for the grinding and adjusting of the safety brake, and this is under heavy load, since the braking moment which is produced for the grinding normally lies far above the loading moment of the lifting gearbox. Another drawback of the known solution is that the take-off shaft of the lifting gearbox, which is configured as a single piece, needs to have a very large diameter at its end face in order to accommodate the cable drum. Since the take-off shaft of the lifting gearbox needs to be made of high-strength material, this solution is very cost intensive.